1. Technical Field
This disclosure relates generally to computer-assisted techniques for creating dental restorations.
2. Brief Description of the Related Art
During the last decade various technological advancements have increasingly started to be applied to systems in the healthcare arena, particularly in dental care. More specifically for example, traditional imaging and computer vision algorithms coupled with soft X-ray sensitive charge coupled device (CCD) based vision hardware have rendered conventional X ray photography ubiquitous, while more advanced data imaging and processing has enabled passive intraoral 3D topography. The latter comprises the acquisition portion of a CAD/CAM system, which would typically be followed by a design step using some sort of manipulating software, and a manufacturing step that might entail an office laser printer-sized milling machine. The entire system allows a dentist to provide a patient the same services a manufacturing laboratory would provide with a certain turnaround time, however, all chair-side and on-the-spot, greatly reducing the possibility of infections and discomfort to the patient. In addition, clinical cases containing raw and processed data are easily shared as digital files between dentists who lack the second portion of the system, i.e. the manufacturing step, and laboratories who have adapted and evolved to embrace CAD/CAM.
In a clinical case where a patient is required a crown, for example, traditionally the dentist would prepare the area, and take a physical (active) impression using a silicone-based agent, thereby subjecting the patient to some discomfort during the process. The next step requires the dentist to place a temporary crown over the area and then schedule the patient for an additional visit once the final crown based on the original impression has been manufactured by a laboratory. During this time, the patient is more subject to local infections. The entire process of mold-taking and re-shaping of materials at the laboratory is involved, is rather cumbersome and outdated, and it contains several steps that must be controlled by tight tolerances.
Intraoral, in-vivo passive 3D scanning is a rather challenging task. A multitude of technical and economic factors impose numerous constraints and add difficulties to the problem. For these reasons, successful systems must address and solve all these challenges, rendering them much more complex than otherwise conceptually simple 3D scanners. First, consider the operating environment, i.e. intraoral on a live patient. Digital imaging complications arise due to the restricted operating volume imposing a certain arrangement of optics and sensors such as to facilitate practical system operation in-vivo and intraoral as a probing device. Further, this environment is dark, contains air with a high degree of relative humidity expunged from the patient's lungs with every breath, and it facilitates artifact contamination of areas of interest by the mere presence of saliva, air bubbles within it and the patient's tongue itself. In addition, the environment is not static, as the patient is not a still unanimated object.
Second, consider the operator, i.e. the dentist. The device must be ergonomically designed around the system to ensure it is a useful tool and can solve the problem. Power consumption and power dissipation are important considerations. Moreover, as a hand-held medical device, it must pass additional regulatory affairs imposed by government authorities, as well as comply with the local standard electromagnetic interference/emission laws.
Third, consider the quality of the data obtained in the scanning process; if not comparable or better with current active (i.e. mold) impression-taking, the whole process is rendered null. The quality and accuracy of the data must also be consistent with the requirements of the CAM step of the process. Ultimately how well a milled restoration fits a patient's preparation area is a function of all of these factors.
There are several commercially-available solutions, including systems that integrate the CAM component. Some solutions still rely on contrast enhancing agents applied as a spray on the preparation area to mitigate some of the difficulties of imaging intra orally in-vivo. The 3D scanning implementations available employ several methods for obtaining surface topography estimations. These range from solutions exploiting depth map generation by confocal imaging, to fringe projection assisted 3D imaging, although other approaches such as correspondence-assisted stereoscopic imaging or plenoptic imaging may be used. Typically, the highest degree of data accuracy and ease of use, coupled with the economics and availability of off the shelf components, is greatly facilitated by employing a structured light projection technique, such as provided by a commercial system such as E4D Dentist, from E4D Technologies, LLC, of Dallas, Tex.